Set parent with automatic weight error
that will happen sometimes and there is no solution but to add weights yourself. you could try with envolope weights too, but that wont always work either. you could try checking your scale rotation and location too. they should be at 0 0 0 for rotation and location, and 1 1 1 for scale. Also looking at your model, you have some complex things like the finger blasters, and I may be wrong, but it also looks really high poly, so that may be an issue.